A. J. Soares headed in the winner in the 89th minute of a game the Impact had dominated to give the New England Revolution a 1-0 victory and stun the 19,988 fans at Saputo Stadium on Saturday afternoon.
''That summed up our season in one play,'' said Montreal midfielder Patrice Bernier. ''A goal late in the game off a set piece.
''It's a bit sad because it's a game where if we had more killer instinct we win 3-0. But because we gave out so much energy, you go at full speed and maybe lose that calmness to just put it in.''
The ninth-place Revolution (9-17-8) ended a 13-game winless run in road matches dating to a March 31 victory in Los Angeles. And they won for the first time on Canadian soil, having previously gone 0-5-3 in Montreal, Toronto and Vancouver.
The Impact (12-16-6) finished seventh in the Eastern Conference and out of the playoffs. It was their league-worst 17th goal allowed in the last 15 minutes of a match.
''We ended up with 42 points, which is a fair amount for an expansion team, but we know we had a chance to have more,'' said midfielder Davy Arnaud. ''In games like this we've let some points slip along the way when we've had good performances.
''But overall I think we can be proud of the work we've done.''
Montreal wanted a win to secure top spot among the three Canadian MLS clubs and be seeded first for next year's Amway Canadian Championship.
For 88 minutes, they did everything they needed to do but score.
They controlled the ball 62 per cent of the time and outshot New England 16-5, including 8-3 in shots on target, but couldn't finish a play.
With regular time nearly ended, the Revolution got a free kick to the left of the Montreal goal, and Soares was inside the box to head Chris Tierney's free kick past the reach of goalkeeper Troy Perkins.
The drumming and singing from the grandstands fell into silence and disbelief, except among a small pocket of New England supporters in the east stands.
''It's frustrating,'' added Arnaud. ''We dominate the game, we have enough chances, but that's soccer sometimes.
''It's tough to swallow because it's one of the most lopsided games I've been a part of and ended up losing. Some days, for whatever reasons, it's not meant to be your day.''
The Impact scored 45 goals in their expansion year, which was better than average, but the attack cooled at the end with only one goal in their last four matches.
It wasn't for lack of chances.
A Marco Di Vaio cross got between goalkeeper Bobby Shuttleworth's legs and rolled along the goal-line, but stayed out as Andrew Wenger failed to react in the 50th minute
Moments later, Wenger won a ball in heavy traffic and fed Di Vaio for another cross that the onrushing Collin Warner couldn't quite get a boot on.
Frustration was evident, and coach Jesse Marsch sent in attacker Justin Mapp for defender Matteo Ferrari in the 70th minute. A minute later, the veteran raced in on the left side and had his shot blocked, then fired another over the bar.
A Di Vaio free kick from 19 yards in front of the goal in the 81st minute got through the wall but was gathered in by Shuttleworth. And a corner with five minutes left in normal time saw Arnaud's close-range shot blocked by defender Blair Gavin.
Even putting veteran striker Eduardo Sebrango in for the final 10 minutes didn't produce a goal for the 39-year-old, who may have played his last match for the club.
The Revolution, also playing their season finale, nearly scored on a counterattack after Montreal fullback Jeb Brovsky fell and Perkins needed to make a close-range save on Fernando Cardenas in the 28th minute.
Before the match, Bernier was handed the Guiseppe Saputo Trophy as the team's player of the year as voted by fans and the media. Bernier led the club with nine goals and eight assists.
''It's a great honour,'' he said. ''But it would have been better if we'd gone to the playoffs and done something more interesting.
''I've won awards before, but being at home it means more in front of friends and family. It's something they can't take back. My name's going to be on the plaque now.''
The Impact won't break up for the off-season just yet. They leave Nov. 5 to play two friendly matches against Italian Serie-A clubs.
Coach Jesse Marsch said finishing off games will be a priority for next season, as will dealing with set pieces, but for now ''the foundation that's been laid down bodes well for the future.''